The "crown" of a tooth is that part that is above the gums.  Sometimes more of the tooth below the gums must be exposed to fill a cavity near the gumline or place a new crown on a broken tooth.  "Crown lengthening" provides more tooth structure for this to to be done.  This helps to ensure that the general dental procedure is successful and prevents gum irritation.

Procedure: The periodontist administers a local anesthetic and gently recontours excess gum and bone tissue to expose more of your natural tooth.

A cavity is below the gum line preventing it from being filled easily. The periodontist gently opens the area.
He then contours the bone to lower the gum line. The bone has been lowered which will lower the level of the gum when it heals.
The area has been closed up and now the cavity is exposed and can be filled. The cavity has been filled and the procedure is done.